context_watch 1.0.2

A hassle-free way to watch for observable value changes within the `build` method.

context_watch

Listen to Listenable (ValueNotifier/AnimationController/ScrollController/TabController etc.) or Stream/ValueStream changes using a simple observable.watch(context). No strings attached.

Getting started

flutter pub add context_watch

Wrap your app in a ContextWatchRoot

ContextWatchRoot(
  child: MaterialApp(...),
);

Usage

final counterNotifier = ValueNotifier(0);

class Example extends StatelessWidget {
  const Example({
    super.key,
    required this.child,
  });

  final Widget child;

  @override
  Widget build(BuildContext context) {
    final counter = counterNotifier.watch(context);
    return Text('Counter: $counter');
  }
}

That's it! No builders, no custom base classes.

Just call watch() on your observable value and you've got a reactive widget. watch()'ing a ValueListenable or a ValueStream will also give you the current value as a result.

It doesn't matter where you call watch() from or how many times you call it. While in a build phase, it will always register the context as a listener to the observable value. Whenever the observable notifies of a change, the context will be marked as needing a rebuild.

Additional information

Performance

Sounds too good to be true? It's not! But it comes at a slight performance cost. My tests show ~2-10% slowdown compared to the equivalent ValueListenableBuilder. Still, it's fast enough for most use cases. Also, as the benchmark shows, the same performance cost applies to other observability solutions like Riverpod or MobX, so it shouldn't be a problem.

context.unwatch()

Unfortunately, InheritedWidgets in Flutter never unsubscribe their dependents. context_watch does everything it can to compensate for that, so that if widget goes from 2 watch()'es to only 1 - the second watch() will be discarded, as expected. But still, there exists a single case where it's not possible to automagically compensate for a Flutter's limitation. If a widget goes from 1 (or more) watch() to 0, the context will still be registered as a listener to all of them, as it's impossible to know that watch()'es are now completely gone from the build method.

So, given this example:

Widget build(BuildContext context) {
  if (condition1) {
    counter1.watch(context);
  }
  if (condition2) {
    counter2.watch(context);
  }
  ...
}

if widget transitions from condition1 == true && condition2 == true to condition1 == false && condition2 == true - it will be rebuilt only when counter2 changes. But if widget transitions from condition1 == true && condition2 == true to condition1 == false && condition2 == false - it will be rebuilt when either counter1 or counter2 changes.

To avoid this, you can manually call context.unwatch() at the beginning of the build method:

Widget build(BuildContext context) {
  context.unwatch();
  if (condition1) {
    counter1.watch(context);
  }
  if (condition2) {
    counter2.watch(context);
  }
  ...
}

This way, all previous watch()'es will be discarded each build and only the new ones will be registered. This comes at a cost, though - my benchmarks show that it's ~50% slower than the default behavior. So, if you're not experiencing any issues with the default behavior - don't use context.unwatch().